Electrical circuits including inductive and capacitive networks (LC networks) are known for use in such functions as creating a time delay for an electrical signal or forming a tank circuit having a specific resonant frequency. Substantially planar, resonant frequency circuits are typically provided for use as detectable tags in antipilferage systems. Presently, such LC networks are formed by such methods as installing discrete inductors and capacitors; or by etching conductive paths in the shape of inductive coils on an insulating substrate and adding either discrete capacitors or capacitive pads on the same substrate (see for example U.S. Pat. No. 4,021,705), both of which are labor intensive, and therefore costly. Because of the prominence of these LC networks within electrical circuits, it is highly desirable to have a low cost, and yet reliable method of manufacture for these networks.
In a co-pending patent application, Ser. No. 256,934, filed Apr. 23, 1981, by one of the present inventors, W. C. Tait, together with Lanny L. Harklau, one such simplified and low-cost method of manufacturing LC networks is disclosed. That method comprises cutting, punching and removing a predetermined pattern from a first conductive sheet, typically a conductive foil. What remains of the conductive sheet after this first cutting operation is a conductive path having an essentially coil-like configuration of windings, with a plurality of interconnections between the adjacent windings which are purposely left by the first cutting operation to add rigidity and thereby cause the conductive sheet to remain planar and essentially self supporting. A similar cutting and removing operation is also performed on a second conductive sheet resulting in a conductive path having an essentially coil-like configuration, and also having a plurality of interconnections between the adjacent windings wherein the conductive sheet remains intact. The self-supporting nature of these conductive paths facilitates their handling in the next method step, when at least one capacitor is formed by orienting and laminating the first and the second conductive sheets to opposing faces of a non-conductive, dielectric sheet with at least one portion of the conductive path on the first conductive sheet substantially aligned with at least one portion of the conductive path on the second conductive sheet, but with the aligned portions separated from each other by the non-conductive dielectric sheet. With the added strength of the non-conductive sheet within the lamination, at least one inductor can now be formed by cutting and removing a predetermined pattern from the lamination, thereby eliminating at least some of the interconnections between the adjacent windings formed by each of the conductive sheets. What remains is at least two continuous conductive paths separated by a non-conductive layer, thus forming the LC network.
Since the LC network invention is entirely formed by simple cutting and laminating operations, the manufacturing cost is kept desirably low.